Eye and head movements must be coordinated to redirect the line of sight ("gaze direction") and to reduce image motion across the retina as the head moves through space. Because these goals are inherently in conflict, the required pattern of coordination is complex. In previous research, eye movements were often used to study vestibular responses (vestibulo-ocular reflex or VOR);experiments were performed in head-restrained subjects undergoing passive rotational stimuli. However, this approach is unable to assess the interplay that naturally occurs between movements of the eye, head, and body and thus is limited in its ability to illuminate the role of the vestibular system in eye-head coordination. In this study, the head is unrestrained and we compare responses to passive whole body rotations (PWBR) and self-generated (active) head movements. PWBR allows for the study of eye and head movements that synergistically contribute to gaze stability. In contrast, during self-generated head movements, compensatory head movements would oppose the animal's intention to redirect the line of sight. Thus, the role of the vestibular system during active head movements must be distinguished from its role during compensatory movements associated with PWBR. The goal of this research is to understand the contribution of the vestibular system during the differing patterns of eye-head coordination provoked by PWBR or self-generated head movements. This goal will be achieved in two specific aims: (1) determine the role of descending vestibular inputs to cervical motoneurons during PWBR and (2) determine the role of descending vestibular inputs to cervical motoneurons during self-generated head movements versus the PWBR condition. Weak galvanic vestibular stimulation (GVS) will be used as an experimental intervention to manipulate the vestibular outflow since this method has been shown to selectively suppress or enhance the vestibular signal from irregular afferents that may preferentially contribute to the vestibulospinal signal. These aims will test the hypothesis that descending vestibular inputs play a significant role in producing the compensatory head movement response during PWBR but may be suppressed during active head movements. Cervical motoneuron activity will be assessed independently of head movement by measuring EMG activity from selected neck muscles involved in the compensatory or active head turns. PUBLIC HEALTH RELEVANCE: Diagnostic tests of inner ear balance mechanisms rely heavily on measurement of eye movements. However, balance is naturally the result of a coordinated interaction of eye, head and body movements;thus consideration of only one of these components may lead to an incomplete understanding of the cause of an inner ear medical problem. This research will be instrumental in the development of novel approaches to clinical tests of inner ear dysfunction as well as its improved treatments.